docs: def maxlevel(lst):
      """Return maximum nesting depth"""
      maxlev = 0
      def f(lst, level):
          nonlocal maxlev
          if isinstance(lst, list):
              level += 1
              maxlev = max(level, maxlev)
              for item in lst:
                  f(item, level)
      f(lst, 0)
      return maxlev